Method of making retrochromic beads and kit thereof

ABSTRACT

The present invention provides a method of making a plurality of intrinsically retrochromic beads. The present invention further provides a kit comprising intrinsically retrochromic beads having different retroreflective colors.

TECHNICAL FIELD

[0001] The present invention relates generally to retroreflective beadsthat cause a change in color of retroreflected light, methods of makingsame, and articles containing same.

BACKGROUND

[0002] “Retroreflectivity” means the ability, if engaged by a beam oflight, to reflect that light substantially back in the direction of thelight source. Films, for example, transparent adhesive-backed overlayfilms, having covert retroreflective patterns have been used toauthenticate security articles (e.g., passports, identification badges).Typically, such overlay films have covert retroreflective patterns thatare not readily legible under conditions of diffuse illumination, butbecome readily legible if viewed in retroreflective mode (e.g., with aflashlight or specialized device such as a retroviewer).

[0003] Depending on the application, the addition of covert color toretroreflective articles may provide additional security features (e.g.,as in the case of authentication of passports or identification badges),or novel visual effects (e.g., as used in the design and production ofgraphic articles).

[0004] As used herein, the term “retrochromism” refers to the ability ofan object, or region of an object, if viewed in retroreflective mode(i.e., with the illumination source substantially co-linear with theline of sight, typically forming an angle of reflection of about 10degrees or less), to exhibit a reflected color different from the colorexhibited if the object or region is viewed in other thanretroreflective mode. Various constructions that exhibit retrochromismare known. For example, in one such construction high refractive indexglass beads are partially embedded in a layer of or layers ofmaterial(s) (e.g., including a dielectric mirror). A change in color ofretroreflected light results from the interaction of light, the bead,and the layer(s) into which it is partially embedded. Such articles maybe prone to loss of, or irregularity in, their retrochromism due tocoating thickness variations of the layer(s) and differences in thedepth of penetration of the beads into the layer(s). Such articles mayfurther be prone to damage caused by flexing the article during usage,or if used as heat-shrink tamper indicating films. Further, it would belaborious and difficult to prepare such retrochromic articles withcomplex patterns of multiple retroreflective colors.

[0005] Such drawbacks could be overcome using retrochromic beads thatare retrochromic, regardless of orientation or position. Thus, it wouldbe desirable to have simple, economical methods for preparing suchintrinsically retrochromic beads in a wide variety of retroreflectivecolors. It would also be desirable to have a kit comprisingintrinsically retrochromic beads wherein the beads could be blended toachieve custom retroreflective colors.

SUMMARY OF THE PRESENT INVENTION

[0006] In one aspect, the present invention provides a method of makingintrinsically retrochromic beads comprising:

[0007] providing a fluidized bed of beads;

[0008] depositing a concentric optical interference layer onto therespective surface of each of a plurality of beads to form a pluralityof coated beads; and

[0009] removing a first portion of coated beads from the fluidized bed,

[0010] wherein the concentric optical interference layers comprising thefirst portion have a first average thickness, and wherein the firstportion comprises intrinsically retrochromic beads.

[0011] In some embodiments, the deposition of the concentric opticalinterference layer can be monitored by viewing the coated beads inretroreflective mode.

[0012] In another aspect, the present invention provides a kitcomprising a first plurality of intrinsically retrochromic beads havinga first average retroreflective color, a second plurality ofintrinsically retrochromic beads having a second average retroreflectivecolor, and a third plurality of intrinsically retrochromic beads havinga third average retroreflective color, wherein said retroreflectivecolors are distinct and comprise a retroreflective color palette.

[0013] In yet another aspect, the present invention provides a method ofpreparing a plurality of retroreflective beads having a customretroreflective color comprising:

[0014] providing a first plurality of intrinsically retrochromic beadshaving a first average retroreflective color;

[0015] providing a second plurality of intrinsically retrochromic beadshaving a second average retroreflective color; and

[0016] combining, with mixing, said first and second pluralities ofretrochromic beads.

[0017] According to the various aspects of the present invention,intrinsically retrochromic beads having a wide range of retrochromiccolors may be easily and economically prepared. Such beads are usefulfor applications including, for example, authentication of securitydocuments and in the graphic arts.

[0018] The following definitions are used throughout the specificationand claims:

[0019] “Intrinsically retrochromic bead” means a bead that exhibitsretrochromism, in substantially all orientations, if immersed in itsentirety in at least one isotropic medium.

[0020] “Light” refers to electromagnetic radiation having one or morewavelengths in the visible (i.e., from about 400 nm to about 700 nm),ultraviolet (i.e., from about 200 nm to about 400 nm), and/or infrared(i.e., from about 700 nm to about 100 micrometers) regions of theelectromagnetic spectrum.

[0021] “Metal oxide” refers to a material made up primarily of one ormore types of metal and/or metalloid cations and oxygen, and which maycontain minor amounts of other elements and compounds.

[0022] “Refractive index” refers to the index of refraction at awavelength of 589.3 nanometers (i.e., nm) corresponding to the sodiumyellow d-line, and a temperature of 20° C., unless otherwise specified.

[0023] “Retrochromic” means that the object being referred to exhibitsretrochromism.

[0024] “Retrochromic pattern” refers to a pattern comprising one or moreretrochromic regions.

[0025] “Retroreflective color” refers to the appearance ofretroreflected light. As used throughout the specification and claims,differences between retroreflective colors are to be determined usingthe same illumination source.

[0026] “Retroreflective color palette” refers to at least threeretroreflective colors, wherein the difference spectra associated withthe retroreflective colors exhibit relative maxima or minima that arespaced apart by at least 20 nanometers in wavelength. “Differencespectra” refers to the difference in the intensity versus wavelengthspectra of the illumination source and the retroreflected light aftereach of said spectra is normalized so that the maximum intensity isdefined to be 100 percent.

[0027] “Viewable” means visually observable at some distance from theobject being viewed.

[0028] “Viewable region” refers-to a portion of an object having aboundary or general extent that is substantially apparent or discernibleto a viewer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a cross-sectional representation of a Type Iretrochromic bead depicting exemplary paths of retroreflected light.

[0030]FIG. 2 is a flow diagram of an exemplary process, according to thepresent invention, for making Type I retrochromic beads.

DETAILED DESCRIPTION

[0031] Intrinsically retrochromic beads may, for example, be of at leasttwo types, referred to herein as Type I and Type II intrinsicallyretrochromic beads.

[0032] Type I Intrinsically Retrochromic Beads

[0033] Referring now to FIG. 1, a Type I intrinsically retrochromic bead100 comprises a transparent substantially spherical core 110 havingthereon a concentric optical interference layer 120 having an exteriorsurface 125. Core 110 contacts optical interference layer 120 atinterface 115.

[0034] Typically, concentric optical interference layer 120 forms asubstantially uniform and complete layer over the entire surface ofspherical core 110. Desirably, the concentric optical interference layeris uniform and complete, however minor imperfections in the layer (e.g.,pinholes and/or minor thickness fluctuations) may be tolerated as longas they are not of sufficient size or amount to render the bead notintrinsically retrochromic.

[0035] Light is typically reflected at interfaces between materialshaving differing refractive indices (e.g., having a difference inrefractive indices of at least 0.1). Thus, a sufficient difference inthe refractive indices of the core 110 and substantially transparentoptical interference layer 120 gives rise to a first reflection atinterface 115. Similarly, a sufficient difference in the refractiveindices of optical interference layer 120 and any background medium(e.g., vacuum, gas, liquid, solid) contacting optical interference layer120 gives rise to a second reflection at exterior surface 125. Throughproper selection of the thickness and refractive index of the opticalinterference layer, the two reflections may optically interfere witheach other, resulting in a retroreflected color different from thatwhich would otherwise be observed in the absence of such interference.

[0036] For example, retrochromic behavior can result from destructiveinterference of a portion of the optical spectrum determined by thethickness and refractive index of the optical interference layer. Thiseffect is visible primarily if viewed in retroreflective mode, and issubstantially not observable if viewed in other than retroreflectivemode.

[0037] Referring again to FIG. 1, light 130 that is incident on Type Iintrinsically retrochromic bead 100 is largely transmitted throughoptical interference layer 120, and enters core 110. A portion of theincident light 130 may be reflected at exterior surface 125 or interface115. Retroreflection results from the portion of light 130 which enterscore 110 and is at least partially focused by refraction onto the backof core 110. As refracted light 135 encounters interface 115 at the backof core 110, some of refracted light 135 is reflected back as reflectedlight 140 towards the front of the bead where it ultimately emerges fromthe bead as retroreflected light 150 in a direction that issubstantially anti-parallel to incident light 130. Similarly, anotherportion of the focused light passes through optical interference layer120 and is reflected back as reflected light 142 at exterior surface125, which forms an interface with whatever medium in which the Type Iintrinsically retrochromic bead 100 is disposed (e.g., gas, liquid,solid, or vacuum). Reflected light 142 ultimately emerges from the beadas retroreflected light 152 in a direction that is substantiallyanti-parallel to incident light 130. Remaining light that is notreflected passes entirely through the intrinsically retrochromic bead.Interference between reflected light 140 and reflected light 142, and inturn retroreflected light 150 and retroreflected light 152, gives riseto a change in color of the retroreflected light. For example,subtraction of wavelengths from the center of the spectrum of incidentwhite light results in retroreflected light with a red-violet hue (i.e.,retrochromism). Slightly thicker optical interference layers subtractlonger wavelengths, resulting in, for example, green or blue-green hues.

[0038] Since reflection at an interface between two materials isdependent on the difference in refractive indices of the two materials,there is no inherent need to use cores and/or the optical interferencelayers comprising either high or low refractive index materials, as longas a sufficient difference in refractive indices is maintained. Thedifference in refractive indices of core 110 and optical interferencelayer 120, and the difference in refractive indices of opticalinterference layer 120 and the medium in which the retrochromic bead isintended to be used should be at least 0.1, desirably at least 0.2, moredesirably at least 0.3, and most desirably at least 0.4. The refractiveindex of optical interference layer 120 may be either greater than orless than the refractive index of core 110. Generally, the choice ofrefractive index, and the corresponding choice of materials used, willbe determined by the specific choice of the medium that contacts theexterior surface 125 in the region where reflection is intended tooccur.

[0039] The refractive indices of core 110, concentric opticalinterference layer 120, and the medium in which the intrinsicallyretrochromic bead is intended to be used are desirably selected so as tocontrol the focal power of the bead and the strength of reflections frominterfaces 115 and 125. Color saturation of retroreflected light istypically maximized if the index of refraction differences at interfaces115 and 125 are balanced (i.e., substantially equal). If index ofrefraction differences at the two interfaces are not balanced,retroreflected light typically appears pale or “washed out.” At the sametime, brightness of retroreflected color is typically maximized ifbalanced index of refraction differences are as large as possible.

[0040] In order to obtain a high level of retroreflectivity, it istypically desirable to select a core 110 having a relatively high indexof refraction, desirably greater than 1.5, more desirably greater than1.8. This allows the incident light to be at least partially focusedonto the back of core 110 (i.e., the side opposite the source ofincident light).

[0041] For example, a glass bead having a diameter in the range of 20 to200 micrometers and refractive index of 1.9 and an air-exposed surfaceis typically a highly efficient retroreflective microlens for incidentvisible light. If a glass bead having a refractive index of 1.9 iscoated with a thin layer of silica (i.e., an optical interference layer)with refractive index of about 1.4, the coated bead in air has a glassbead-silica layer interface with a refractive index difference of about0.5 and a silica layer-air interface with an refractive index differenceof about 0.4. Such coated beads can produce strong color effects ifviewed using a retroviewer. However, if such coated beads are partiallyembedded in an organic material having an index of refraction muchcloser to that of the silica layer, the index difference at the silicalayer-background medium interface becomes very low and color saturationand retroreflected intensity are dramatically reduced.

[0042] An optical interference layer comprising a high index ofrefraction such as titania can be used to provide significant indexdifferences at both interfaces for this type of construction. Multilayercoatings can also be used to adjust the interference effect, or tosimply fix the index differences of the interfaces (e.g., by using amuch thicker outer coating).

[0043] The thickness of the concentric optical interference layer can,desirably, be selected to produce a pre-determined color effect.

[0044] Light that is reflected at an interface may be reflected with orwithout a phase inversion. Light that passes through a medium having ahigher index of refraction and encounters an interface with a mediumhaving a lower index of refraction will be reflected without phaseinversion. By contrast, light that passes through a medium having alower index of refraction and encounters an interface with a mediumhaving a higher index of refraction will be reflected with phaseinversion. Consequently, the appropriate choice of optical interferencelayer thickness will depend on the refractive index of core 110, therefractive index of the optical interference layer 120, and therefractive index of intended medium in which it is to be disposed. Inany case, the thickness should be selected such that the reflected lightfrom exterior surface 125 is π radians (i.e., 180°) out of phase withlight of the same wavelength reflected from interface 115.

[0045] If the refractive index of the bead is greater than therefractive index of the optical interference layer, which in turn isgreater than the refractive index of the medium, destructiveinterference typically occurs, for example, if the optical interferencelayer 120 has an optical thickness (i.e., thickness divided byrefractive index) that is an odd multiple of one quarter of thewavelength (i.e., quarter wave) to be subtracted. By way ofillustration, for such a relationship of refractive indices, an opticalinterference layer thickness of 137.5 nm may result in a red-violet hueif viewed in retroreflective mode using white light illumination. As theoptical interference layer thickness increases, the destructive centermoves toward longer wavelengths, and blue, blue-green, and yellow-greenretroreflected colors are progressively observed.

[0046] If the refractive index of the bead is less than the refractiveindex of the optical interference layer, which in turn is greater thanthe refractive index of the medium, destructive interference typicallyoccurs if the optical interference layer 120 has an optical thicknessthat is a multiple of one half of the wavelength to be subtracted.

[0047] Although the number of possible permutations of refractiveindices, extra layers, and layer thicknesses is quite large, thespecific choice of refractive indices and thicknesses will be readilyapparent to one of ordinary skill in the art upon review of the forgoingdiscussion.

[0048] Type I intrinsically retrochromic beads may be conveniently, andeconomically, prepared using a fluidized bed of transparent beads andvapor deposition techniques. In general, the processes of depositingvapor phase materials onto a fluidized (i.e., agitated) bed of aplurality of beads, as used herein, can be collectively referred to as“vapor deposition processes” in which a concentric layer is deposited onthe surface of respective transparent beads from a vapor form. In someembodiments, vapor phase precursor materials are mixed in proximity tothe transparent beads and chemically react in situ to deposit a layer ofmaterial on the respective surfaces of the transparent beads. In otherembodiments, material is presented in vapor form and deposits as a layeron the respective surfaces of the transparent beads with essentially nochemical reaction.

[0049] Depending upon the deposition process being used, precursormaterial(s) (in the case of a reaction-based deposition process) orlayer material(s) (in the case of a non-reaction-based process),typically in vapor phase, is or are placed in a reactor with transparentbeads. The present invention desirably utilizes a vapor phase hydrolysisreaction to deposit a concentric optical interference layer (e.g., alayer of metal oxide) onto the surface of a respective core. Suchprocess is sometimes referred to as a chemical vapor deposition (“CVD”)reaction.

[0050] Desirably, a low temperature, atmospheric pressure chemical vapordeposition (“APCVD”) process is used. Such processes do not requirevacuum systems and can provide high coating rates. Hydrolysis-basedAPCVD (i.e., APCVD wherein water reacts with a reactive precursor) ismost desired because of the ability to obtain highly uniform layers atlow temperatures, e.g., typically well below 300° C.

[0051] The following is an illustrative vapor phase hydrolysis-basedreaction:

TiCl₄+2H₂O→TiO₂+4HCl

[0052] In the illustrative reaction, water vapor and titaniumtetrachloride, taken together, are considered metal oxide precursormaterials.

[0053] Useful fluidized bed vapor deposition techniques are described,for example, in U.S. Pat. No. 5,673,148 (Morris et al.), the disclosureof which is incorporated herein by reference.

[0054] A well-fluidized bed can ensure that uniform layers are formedboth for a given particle and for the entire population of particles. Inorder to form substantially continuous layers covering essentially theentire surfaces of the transparent beads, the transparent beads aresuspended in a fluidized bed reactor. Fluidizing typically tends toeffectively prevent agglomeration of the transparent beads, achieveuniform mixing of the transparent beads and reaction precursormaterials, and provide more uniform reaction conditions, therebyresulting in highly uniform concentric optical interference layers. Byagitating the transparent beads, essentially the entire surface of eachassembly is exposed during the deposition, and the assembly and reactionprecursors or layer material may be well intermixed, so thatsubstantially uniform and complete coating of each bead is achieved.

[0055] If using transparent beads that tend to agglomerate, it isdesirable to coat the transparent beads with fluidizing aids, e.g.,small amounts of fumed silica, precipitated silica, methacrylato chromicchloride having the trade designation “VOLAN” (available from Zaclon,Inc., Cleveland, Ohio). Selection of such aids and of useful amountsthereof may be readily determined by those with ordinary skill in theart.

[0056] One technique for getting precursor materials into the vaporphase and adding them to the reactor is to bubble a stream of gas,desirably a non-reactive gas, referred to herein as a carrier gas,through a solution or neat liquid of the precursor material and theninto the reactor. Exemplary carrier gases include argon, nitrogen,oxygen, and/or dry air.

[0057] Optimum flow rates of carrier gas(es) for a particularapplication typically depend, at least in part, upon the temperaturewithin the reactor, the temperature of the precursor streams, the degreeof assembly agitation within the reactor, and the particular precursorsbeing used, but useful flow rates may be readily determined by routineoptimization techniques. Desirably, the flow rate of carrier gas used totransport the precursor materials to the reactor is sufficient to bothagitate the transparent beads and transport optimal quantities ofprecursor materials to the reactor.

[0058] Referring to FIG. 2, wherein an exemplary process for makingintrinsically retrochromic beads is shown, carrier gas 202 is bubbledthrough water bubbler 204, to produce water vapor-containing precursorstream 208. Carrier gas 202 is also bubbled through titaniumtetrachloride bubbler 206, to produce titanium tetrachloride-containingprecursor stream 230. Precursor streams 208 and 230 are then transportedinto reactor 220. Cores 110 are introduced into reactor 220, andintrinsically retrochromic beads 100 are removed therefrom.

[0059] Typically, precursor flow rates are adjusted to provide anadequate deposition rate and to provide a metal oxide layer of desiredquality and character. Desirably, flow rates are adjusted such that theratios of precursor materials present in the reactor chamber promotemetal oxide deposition at the surface of the transparent beads withminimal formation of discrete, i.e., free floating, metal oxideparticles, elsewhere in the chamber. For example, if depositing layersof titania from titanium tetrachloride and water, a ratio of betweenabout eight water molecules per each titanium tetrachloride molecule toone water molecule per two titanium tetrachloride molecule is generallysuitable, with about two water molecules of water per titaniumtetrachloride molecule being preferred. Under these conditions there issufficient water to react with most of the titanium tetrachloride andmost of the water is adsorbed onto the surface of the retroreflectivebead. Much higher ratios tend to yield substantial quantities ofnon-adsorbed water that might result in formation of oxide particulatesrather than the desired oxide layers.

[0060] Desirably, precursor materials have sufficiently high vaporpressures that sufficient quantities of precursor material will betransported to the reactor for the hydrolysis reaction and layerdeposition process to proceed at a conveniently fast rate. For instance,precursor materials having relatively higher vapor pressures typicallyprovide faster deposition rates than precursor materials havingrelatively lower vapor pressures, thereby enabling the use of shorterdeposition times. Precursor sources may be cooled to reduce vaporpressure or heated to increase vapor pressure of the material. Thelatter may necessitate heating of tubing or other means used totransport the precursor material to the reactor, to prevent condensationbetween the source and the reactor. In many instances, precursormaterials will be in the form of neat liquids at room temperature. Insome instances, precursor materials may be available as sublimablesolids.

[0061] Precursor materials that are capable of forming dense metal oxidecoatings via hydrolysis reactions at temperatures below about 300° C.,and typically below about 200° C., are desired for coating glass beads.Desirably, titanium tetrachloride and/or silicon tetrachloride, andwater are used as precursor materials. In addition to volatile metalchlorides, useful precursor materials include, for example, mixtures ofwater and at least one of: metal alkoxide(s) (e.g., titaniumisopropoxide, silicon ethoxide, zirconium n-propoxide), metal alkyl(s)(e.g., trimethylaluminum, diethylzinc). It may be desirable to utilizeseveral precursors simultaneously in a coating process.

[0062] Desirably, mutually reactive precursor materials, e.g., TiCl₄ andH₂O, are not mixed prior to being added to the reactor in order toprevent premature reaction within the transport system. Accordingly,multiple gas streams into the reaction chamber are typically provided.

[0063] Vapor deposition processes include hydrolysis based CVD and/orother processes. In such processes, the beads are typically maintainedat a temperature suitable to promote effective deposition and formationof the concentric optical interference layer with desired properties onthe beads. Increasing the temperature at which the vapor depositionprocess is conducted typically yields a resultant concentric layer thatis denser and retains fewer fugitive unreacted precursors. Sputtering orplasma-assisted chemical vapor deposition processes, if utilized, oftenrequire minimal heating of the article being coated, but typicallyrequire vacuum systems, and can be difficult to use if coatingparticulate materials such as small glass beads.

[0064] Typically, a deposition process that operates at a temperaturelow enough not to undesirably degrade the transparent beads should beselected. Thus, deposition of the optical interference layer isdesirably achieved using a hydrolysis-based APCVD process attemperatures below about 300° C., more desirably below about 200° C.

[0065] Titania and titania-silica layers deposited from tetrachloridesare particularly desired, and are easily deposited by APCVD at lowtemperatures, e.g., between about 120° C. and about 160° C.

[0066] Typically, any dimensionally stable substantially sphericaltransparent bead may be used as a core in practice of the presentinvention. Cores may be inorganic, polymeric or other provided that theyare substantially transparent to at least one wavelength, desirably allwavelengths, of visible light. Typically, cores have a diameter of fromabout 20 to about 500 micrometers, desirably from about 50 to about 100micrometers, although other diameters are possible.

[0067] Desirably, cores comprise a material, desirably an inorganicglass comprising silica, having a refractive index of from about 1.5 toabout 2.5 or even higher, desirably from about 1.7 to about 1.9. Coresmay also have a lower refractive index value depending on the particularintended application, and the composition of the concentric opticalinterference layer. For example, a silica glass bead with refractiveindex as low as about 1.50 may be desirably used as a core because ofthe low cost and high availability of soda-lime-silica (i.e., windowglass). Optionally, cores may further comprise a colorant. Desirably,cores comprise glass.

[0068] Exemplary materials that may be utilized as a core includeglasses (e.g., mixtures of metal oxides such as SiO₂, B₂O₃, TiO₂, ZrO₂,Al₂O₃, BaO, SrO, CaO, MgO, K₂O, Na₂O); and solid, transparent,non-vitreous, ceramic particles as described in, for example, U.S. Pat.Nos. 4,564,556 (Lange) and 4,758,469 (Lange), the disclosures of whichare incorporated herein by reference.

[0069] Exemplary useful colorants include transition metals, dyes,and/or pigments, and are typically selected according to compatibilitywith the chemical composition of the core, and the processing conditionsutilized.

[0070] The concentric optical interference layer employed in practiceaccording to the present invention may be of any transparent materialhaving a different refractive index than the core supporting the layer.Desirably, the concentric optical interference layer should besufficiently smooth so as to be optically clear. Desirably, theconcentric optical interference layer is tough, and not easily chippedor flaked.

[0071] Desirably, the concentric optical interference layer comprises ametal oxide. Exemplary metal oxides useful for the concentric opticalinterference layer include titania, alumina, silica, tin oxide,zirconia, antimony oxide, and mixed oxides thereof. Desirably, theoptical interference layer comprises one of the following: titaniumdioxide, silicon dioxide, aluminum oxide, or a combination thereof.Titania and titania/silica layers are most desired, as they are readilydeposited and form durable layers.

[0072] Advantageously, portions of retrochromic beads having variousoptical interference layer thicknesses and retroreflective colors can beremoved from a reactor sequentially. One, two, three, or morepluralities of retrochromic coated beads, each plurality having adifferent retroreflective color and collectively comprising aretroreflective color palette, may thus be easily obtained by charging areactor with a large quantity of beads and sequentially removing two,three, or more portions of beads during a continued coating run, eachportion comprising beads and having a larger average opticalinterference layer thickness than the preceding portion.

[0073] In one desirable embodiment, the progress of layer deposition maybe monitored by viewing the beads in retroreflective mode, for example,by using a retroviewer (e.g., as described in U.S. Pat. Nos. 3,767,291(Johnson) and 3,832,038 (Johnson), the disclosures of which areincorporated herein by reference) either in situ using a glass-walledreactor or by removal from the reactor. Retroviewers useful for viewingintrinsically retrochromic beads and articles containing them are alsoreadily commercially available, for example, under the trade designation“3M VIEWER” from 3M Company, St. Paul, Minn.

[0074] Type II Intrinsically Retrochromic Beads

[0075] Intrinsically retrochromic beads of Type II comprise partiallytransparent beads having microcrystalline regions therein. Themicrocrystalline regions are of a size typically less than thewavelength range of visible light. The microcrystalline regions scatterlight of different wavelengths in the visible spectrum to differentdegrees. The microcrystalline regions scatter light on the short end ofthe visible wavelength spectrum more effectively than they scatter lighton the long end of the visible wavelength spectrum. If the shorterwavelength components of the incident spectrum are preferentiallyscattered, the longer wavelength components are transmitted within thebead and, ultimately retroreflected. Typically, the microcrystallineregions have a size in the range of from about 10 to about 500 nm.Desirably, the microcrystalline regions have a size of from about 50 to250 nm, with substantially no microcrystalline regions being larger than250 nm.

[0076] The retroreflected spectrum is modified, as compared with theincident spectrum. Modification of the spectrum refers to a changeimparted to the relative intensities of the different incidentwavelengths. Typically, for incident light that is white, colorless TypeII retrochromic beads retroreflect light that is essentially yellow,orange, or red. Light-scattering inside the bead causes a modificationof the retroreflected spectrum as compared with the incident spectrum.For crystallites grown to the size range of 50 to 250 nm, yellow,orange, or red light is transmitted within the bead and eventuallyretroreflected, while light of wavelengths near the violet end of thevisible spectrum is preferentially scattered from the retroreflectivelight path. Preferential scattering of wavelengths near the violet endof the visible spectrum causes their subtraction from the incidentspectrum, and thus they are not retroreflected.

[0077] In one exemplary method, Type II intrinsically retrochromic beadscan be made as follows. First, an aqueous slurry is prepared fromparticulate components comprising at least two metal oxides. Optionally,an inorganic oxide dopant that imparts color to the finished bead bylight absorption (e.g., a colorant) may also be present in the slurry.Desirably, particulate components of the slurry should be chosen suchthat upon melting and rapid quenching an amorphous transparent bead isformed. The transparent bead may contain small crystals (e.g., less than10 nm), but the composition should be selected such that it crystallizesslowly enough that it can be quenched to form a glassy material.

[0078] Metal oxides useful for forming Type II intrinsicallyretrochromic beads are well known in the art. Exemplary metal oxidesinclude SiO₂, Al₂O₃, B₂O₃, Na₂O, K₂O, CaO, MgO, BaO, SrO, TiO₂, ZrO₂.Certain metal oxides are known as strong glass-formers (e.g., SiO₂,GeO₂, As₂O₅, P₂O₅, B₂O₃). A strong glass-former is a metal oxide thatcan be relatively easily quenched into a solid amorphous state from themelt. Desirably, particulate components of the slurry comprise a strongglass-former. Metal oxides that do not readily form glasses bythemselves may be incorporated into Type II intrinsically retrochromicbeads, if used in combination with certain species (known asintermediates) and strong glass-formers. Exemplary intermediates includeAl₂O₃ and PbO. Al₂O₃ may be incorporated into Type II intrinsicallyretrochromic beads if, for example, it is added to a strong glass-former(e.g., SiO₂) in combination with a modifier metal oxides. Such modifiersinclude, for example, alkali metal oxides and alkaline earth oxides(e.g., Na₂O, K₂O, MgO, CaO, SrO, BaO).

[0079] Certain metal oxides, (e.g., TiO₂ ZrO₂) may serve to nucleatecrystallization if included in glass compositions. Such metal oxides areuseful, for example, as nucleating agents for subsequent crystallizationof the glass with heat-treatment.

[0080] Advantageously, flame-forming processes used to form Type IIintrinsically retrochromic beads allow the use of a wider range of glasscompositions than could be formed by traditional processes for thefabrication of glass articles. For example, compositions high in TiO₂and ZrO₂ (e.g., greater than about 50 weight percent) would typically beconsidered inappropriate, as oxide melts which are very high in TiO₂and/or ZrO₂ tend to form crystals during cooling. However, in thepreparation of Type II intrinsically retrochromic beads, it is desirableto include at least one nucleating agent in the aqueous slurry such as,for example, titanium oxide or zirconium oxide.

[0081] As an additional feature, the rapid quench rates that are typicalof the flame-forming process for manufacturing glass beads enable thepreparation of a wider range of compositions as glasses than could beformed using slower cooling rate processes. In particular, eutecticcompositions of traditionally non-glass-forming oxides can be preparedas glass beads. Thus, it is desirable that particulate metal oxidecomponents in the slurry be present in approximately eutecticproportions.

[0082] The fact that compositions generally not expected to form glassescan be formed as glass beads opens up the range of compositions that arepotentially useful for the formation of Type II microcrystallineretrochromic beads. Accordingly, the range of useful compositions is notlimited to the compositions generally regarded as convenient for formingglasses or even glass-ceramics.

[0083] Desirably, microcrystalline retrochromic beads include aglass-forming metal oxide. For example, SiO₂ can aid in initialglass-forming, if included in the bead in an amount, for example, in therange of from about 2 to about 40 percent by weight based on the totalweight of the bead. To generate a high density of microcrystals duringheat-treatment, nucleating agents (e.g., TiO₂ and/or ZrO₂) are useful atlevels ranging from greater than about 5 but less than 80 percent byweight based on the total weight of the bead. TiO₂ and ZrO₂ are alsouseful for achieving a high index of refraction (1.8-2.3). A high indexof refraction is useful for strong retroreflection.

[0084] Additional constituents that may be present in microcrystallineintrinsically retrochromic beads include, for example, B₂O₃, Al₂O₃, MgO,CaO, BaO, Na₂O, and K₂O. The alkali metal oxides and alkaline earthoxides are especially useful for reducing the melting temperature of thecomposition, and can be included at a combined alkali metal oxide andalkaline earth oxide content of up to about 25 percent by weight of thetotal combined weight of the microcrystalline intrinsically retrochromicbead.

[0085] Desirably, if high levels (i.e., >60 percent by weight based onthe total weight of the bead) of TiO₂ are included in the Type IIintrinsically retrochromic bead composition, alkaline earth oxides areincluded at levels of greater than about 10 percent by weight based onthe total weight of the bead, and aid in quenching transparent beadsduring the flame-forming process.

[0086] The slurry is then typically milled and dried to form a powdercake, then ground into particles. Particles are fed into the flame of ahydrogen/oxygen torch where they melt and form intermediate beads asdescribed in, for example, U.S. Pat. No. 6,245,700 (Budd et al.), thedisclosure of which is incorporated herein by reference. Intermediatebeads are rapidly cooled (i.e., quenched), for example in a water tankto form vitreous beads. Cooled vitreous beads may, optionally, be passeda second time through the hydrogen torch to improve their transparency.

[0087] Next, cooled vitreous beads are placed into a crucible (e.g., analumina crucible), and heated in a furnace by slowly ramping thetemperature (e.g., at a rate of 10° C./minute) up to a temperaturesufficient to devitrify them. The elevated temperature should be highenough to cause devitrification of the vitreous beads, but not so highthat the beads fuse together. Desirably the elevated temperature is inthe range of from about 400° C. to about 1200° C., more desirably fromabout 700° C. to about 1100° C. Elevated temperature is maintained for asufficient time to substantially devitrify the beads, desirably a periodranging from about 15 to about 120 minutes. Subsequently, thetemperature is reduced back to room temperature.

[0088] Desirably, the amount of colorant is in the range of from about0.01 to about 5 weight percent, and more desirably in the range of fromabout 0.5 to about 3 weight percent, based on the total weight of themetal oxide components.

[0089] Appropriate processing conditions for generating Type IIintrinsically retrochromic beads from transparent beads having a givenchemical composition can be readily determined as follows.

[0090] In a first procedure, separate portions of beads areheat-treated, for example, by placing them into a furnace with a ramprate of 10° C./min up to a number of equally spaced temperatures rangingfrom room temperature to the melting point of the composition. Suchspacing between heat-treatment temperatures may be, for example, 50° C.or 100° C. Once each portion of beads reaches its desired temperature(i.e., soak temperature), it is maintained at that temperature for aperiod of time that is identical for all portions, for example, onehour. The portions are removed from the furnace and cooled to roomtemperature. With increasing soak temperature, for compositions thatcrystallize before melting, there is typically a progression fromtransparency to opacity for the heat-treated beads.

[0091] If none of the processed portions of the first procedure displaythe desired retrochromic effect, the procedure is repeated with soaktemperatures ranging from the highest temperature at which transparentbeads were observed to the lowest temperature at which opaque beads wereobserved. In this second procedure, soak temperatures spaced by, forexample, 5° C. or 10° C., produce a series of heat-treated bead samplesthat more closely resolve the transition between a state of hightransparency and a state of opacity.

[0092] If none of the processed portions of the second procedure displaythe desired retrochromic effect, a third procedure carried out with soaktemperatures ranging from the highest temperature at which transparentbeads were observed to the lowest temperature at which opaque beads wereobserved in the second procedure. In this third procedure, soaktemperatures are spaced by, for example, 1° C. or 2° C.

[0093] Once appropriate conditions are found, the length of the soaktime may be further adjusted in order to finely control the size of themicrocrystalline regions.

[0094] Regardless of whether intrinsically retrochromic beads are ofType I or Type II, the magnitude of the retrochromic effect typicallydepends on the spectral breadth of source used to illuminate theretrochromic bead. Desirably, the source has a broad spectrum (e.g.,white light), although narrower spectral ranges may also be used.

[0095] Whichever type(s) of intrinsically retrochromic bead(s) is/areused in practice of the present invention, the magnitude ofretroreflection may be increased by coating onto the intrinsicallyretrochromic bead an integral hemispherical reflector as described in,for example, U.S. Pat. No. 2,963,378 (Palmquist et al.), the disclosureof which is incorporated herein by reference.

[0096] Advantageously, pluralities of intrinsically retrochromic beadshaving different retroreflective colors may be combined with a holder toform a kit. Desirably, the retroreflective colors are chosen such thatthey form a retroreflective color palette if viewed using white lightillumination. In some embodiments, the retroreflective color paletteincludes at least three of violet, blue, green, orange, yellow, red, orgreen retroreflective colors if viewed using white light illumination.In some embodiments, the retroreflective color palette may compriseyellow, magenta, and cyan retroreflective colors, if viewed using whitelight illumination.

[0097] The kit may comprise, for example, pluralities of differentintrinsically retrochromic beads corresponding to differentretroreflective colors that are collectively, and separately, containedwithin at least one reclosable container. For example, a box may havecompartments in which individual pluralities of intrinsicallyretrochromic beads having different retroreflective colors are confined.

[0098] In some embodiments, pluralities of intrinsically retrochromicbeads may be confined in separate containers, desirably reclosablecontainers, and assembled as a group with a holder, desirably a holderadapted to receive the separate containers. Exemplary holders include abox, a blister pack, a rack (e.g., a test tube rack), and a bag.

[0099] In some embodiments of the present invention, intrinsicallyretrochromic beads having different retroreflective colors may becombined to form a plurality of beads having a custom retroreflectivecolor, if viewed at relatively low resolution where individual beadscannot be resolved. However, if a plurality of beads having such acustom retroreflective color is viewed at relatively highermagnification where individual beads can be resolved, it will appear asa mixture of beads having different retroreflective colors. Thus, anextra level of covert security may be imparted, as described above, if aplurality of beads having such a custom color is used for authenticationof security documents.

[0100] Intrinsically retrochromic beads, alone or in kit form, areuseful, for example, for preparing articles having a retrochromicpattern.

[0101] The following examples illustrate specific embodiments of thepresent invention. These examples are not intended to limit the presentinvention that is defined in the attached claims.

EXAMPLES

[0102] In the examples that follow, observed retroreflective color wasdetermined by looking through a retroreflective viewer having the tradedesignation “3M VIEWER”.

[0103] In the Examples and Tables that follow:

[0104] “mL” means milliliter;

[0105] “rt” means room temperature (i.e., approximately 20° C.);

[0106] “min” means minute.

[0107] General Procedure for the Preparation of Type I Beads

[0108] The preparation of the silica-coated glass beads of Examples1-182 were carried out using an apparatus similar to that shown in FIG.2. Eighty grams (g) of glass beads with an index of refraction of 1.9and with an average diameter of about 65 micrometers (available underthe trade designation “FLEX-O-LITE 831 SIGN BEADS” from Flex-O-Lite,Inc., Chesterfield, Mo.,) were charged into a glass frit funnel typefluidized bed CVD reactor with a 30 millimeters (i.e., mm) innerdiameter reactor (as described, for example, in Example 1 of U.S. Pat.No. 5,673,148 (Morris et al.), the disclosure of which is incorporatedherein by reference). For examples in which the reaction temperature was50° C. or more, the reactor was wrapped with electric heating tape andmonitored by the use of a thermocouple in the fluidized bed. Forexamples in which the reaction temperature was room temperature (i.e.,approximately 20° C.), no heating tape was used. The bed of beads wasfluidized with a stream of nitrogen gas introduced into the reactorthrough the glass frit (i.e., from the bottom of the bed of beads).Water vapor was simultaneously introduced into the reactor, through theglass frit, in a stream of nitrogen carrier gas by bubbling the carriergas through water in a chamber separate from the reactor.

[0109] The metal oxide precursor compounds, either SiCl₄ (Examples1-174), or a mixture of SiCl₄ and tetraethyl orthosilicate (TEOS,Examples 175-182), were introduced into the reactor, through a glasstube extending downward into the fluidized bed of beads, in a stream ofnitrogen carrier gas by bubbling the carrier gas through the liquidprecursor compound in a chamber separate from the reactor. For Examples175-182, two separate chambers, one for each of the liquid precursorcompounds, were used. Deposition of concentric coatings on the glassbeads in Examples 1-182 commenced when the flow of reactant-ladennitrogen carrier gas through the reactor began.

[0110] Samples of concentrically coated glass beads were periodicallyremoved from the reactor and were evaluated by viewing the samples inretroreflective mode. Thickness of the concentric coating was alsodetermined by examining fractured concentrically coated glass beads witha scanning electron microscope.

[0111] Experimental details such as the flow rates of the reactant-ladencarrier gases, the resultant thickness of the concentric coatings on theglass beads and the retroreflective color of the coated beads arereported in Tables 1 and 2 (below).

[0112] The preparation of the titania-coated glass beads of Examples183-191 was carried out by the procedure used for Examples 1-182, exceptthat a reactor having an 80 mm inner diameter was used and the titaniaprecursor compound was TiCl₄. Glass beads (1800 g) with an index ofrefraction of 1.9 and an average diameter of about 65 micrometers(available under the trade designation “FLEX-O-LITE 831 SIGN BEADS”,from Flex-O-Lite, Inc., Chesterfield, Mo.) were charged into thereactor. The reactor was wrapped with electric heating tape, which wasused to maintain the temperature of the fluidized bed at approximately175° C. as measured by a thermocouple in the fluidized bed. The flowrate of the nitrogen carrier gas through each of the separate reactantchambers was 7 liters per minute.

[0113] Samples of concentrically coated glass beads were periodicallyremoved from the reactor and evaluated by viewing the samples inretroreflective mode with a retroviewer as above. Thickness of thedeposited concentric coating (i.e., optical interference layer) was alsodetermined by examining fractured concentrically coated glass beads witha scanning electron microscope. Deposition times and the resultantretroreflective colors of the titania-coated glass beads, visuallyobserved using a 3M VIEWER, are reported in Table 3 (below). TABLE 1Reaction SiCl₄ Flow H₂O Flow N₂ Flow Reaction Coating Observed ExampleTemperature Rate Rate Rate Time Thickness Retroreflective No. (° C.)(mL/min) (mL/min) (mL/min) (min) (nm) Color 1 rt 25 800 800 10 31.3light gray 2 rt 25 800 800 20 62.6 light tan 3 rt 25 800 800 30 93.9blue 4 rt 25 800 800 40 125.2 faint green-gray tint 5 50 42 475 150 47.4 white 6 50 42 475 150 5 9.2 white 7 50 42 475 150 6 11.0 white 8 5042 475 150 7 12.9 very faint gray tint 9 50 42 475 150 8 14.7 very faintgray tint 10 50 42 475 150 9 16.6 very faint gray tint 11 50 42 475 15010 18.4 very faint gray tint 12 50 42 475 150 11 20.2 very faint graytint 13 50 42 475 150 12 22.1 very faint gray tint 14 50 42 475 150 1323.9 very faint gray tint 15 50 42 475 150 14 25.8 very faint gray tint16 50 42 475 150 15 27.6 very faint gray tint 17 50 42 475 150 16 29.4light gray 18 50 42 475 150 17 31.3 light gray 19 50 42 475 150 18 33.1light gray 20 50 42 475 150 19 35.0 light gray 21 50 42 475 150 20 36.8light gray 22 50 42 475 150 21 38.6 light gray 23 50 42 475 150 22 40.5light gray 24 50 42 475 150 23 42.3 light gray 25 50 42 475 150 24 44.2light gray 26 50 42 475 150 25 46.0 light gray 27 50 42 475 150 26 47.8very faint tan tint 28 50 42 475 150 27 49.7 very faint tan tint 29 5042 475 150 28 51.5 very faint tan tint 30 50 42 475 150 29 53.4 fainttan tint 31 50 42 475 150 30 55.2 faint tan tint 32 50 42 475 150 3157.0 light tan 33 50 42 475 150 32 58.9 light tan 34 50 42 475 150 3360.7 light tan 35 50 42 475 150 34 62.6 light tan 36 50 42 475 150 3564.4 light tan 37 50 42 475 150 36 66.2 light tan-faint red tint 38 5042 475 150 37 68.1 faint red tint 39 50 42 475 150 38 69.9 faint redtint 40 50 42 475 150 39 71.8 light red 41 50 42 475 150 40 73.6red-violet 42 50 42 475 150 41 75.4 violet-red 43 50 42 475 150 42 77.3light violet-red 44 50 42 475 150 43 79.1 light violet 45 50 42 475 15044 81.0 violet 46 50 42 475 150 45 82.8 violet 47 50 42 475 150 46 84.6violet-blue 48 50 42 475 150 47 86.5 blue-violet 49 50 42 475 150 4888.3 light blue-violet 50 50 42 475 150 49 90.2 light blue 51 50 42 475150 50 92.0 blue 52 50 42 475 150 51 93.8 blue-green 53 50 42 475 150 5295.7 light blue-green 54 50 42 475 150 53 97.5 light blue-green 55 50 42475 150 54 99.4 light green-blue 56 50 42 475 150 55 101.2 lightgreen-blue 57 50 42 475 150 56 103.0 light green-blue 58 50 42 475 15057 104.9 light green-gray 59 50 42 475 150 58 106.7 light green-gray 6050 42 475 150 59 108.6 light green-gray 61 50 42 475 150 60 110.4 faintgreen-gray tint 62 50 42 475 150 61 112.2 faint green-gray tint 63 50 42475 150 62 114.1 faint green-gray tint 64 50 42 475 150 63 115.9 faintgreen-gray tint 65 50 42 475 150 64 117.8 faint green-gray tint 66 50 42475 150 65 119.6 faint green-gray tint 67 50 42 475 150 66 121.4 faintgreen-gray tint 68 50 42 475 150 67 123.3 faint green-gray tint 69 50 42475 150 68 125.1 faint green-gray tint 70 50 42 475 150 69 127.0 veryfaint gray tint 71 50 42 475 150 70 128.8 very faint gray tint 72 50 50500 250 5 9.0 very faint gray tint 73 50 50 500 250 10 18.0 very faintgray tint 74 50 50 500 250 15 27.0 light gray 75 50 50 500 250 20 36.0light gray 76 50 50 500 250 25 45.0 light gray 77 50 50 500 250 30 54.0light tan 78 50 50 500 250 35 63.0 light tan-very faint red tint 79 5050 500 250 40 72.0 light red 80 50 50 500 250 55 110.0 faint green-graytint 81 50 50 500 250 60 120.0 faint green-gray tint 82 50 42 475 150 2543.3 light gray 83 50 42 475 150 29 50.2 faint tan tint 84 50 42 475 15033 57.1 light tan 85 50 42 475 150 35 60.6 light tan 86 50 42 475 150 3764.0 light tan-very faint red tint 87 50 42 475 150 39 67.5 faint redtint 88 50 42 475 150 41 70.9 red-violet 89 50 42 475 150 43 74.4violet-red 90 50 42 475 150 45 77.9 light violet 91 50 42 475 150 4781.3 violet 92 50 42 475 150 49 84.8 violet-blue 93 50 42 475 150 5188.2 light blue-violet 94 50 50 500 250 10 19.1 very faint gray tint 9550 50 500 250 15 28.7 light gray 96 50 50 500 250 20 38.2 light gray 9750 50 500 250 25 47.8 very faint tan tint 98 50 50 500 250 30 57.3 lighttan 99 50 50 500 250 35 66.9 faint red tint 100 50 50 500 250 36 68.8faint red tint 101 50 50 500 250 37 70.7 light red 102 50 50 500 250 3872.6 red-violet 103 50 50 500 250 39.5 75.4 violet-red 104 50 50 500 25010 18.4 very faint gray tint 105 50 50 500 250 15 27.6 very faint graytint 106 50 50 500 250 20 36.8 light gray 107 50 50 500 250 25 46.0light gray 108 50 50 500 250 27 49.7 very faint tan tint 109 50 50 500250 29 53.4 faint tan tint 110 50 50 500 250 30 55.2 faint tan tint 11150 50 500 250 31 57.0 light tan 112 50 50 500 250 32 58.9 light tan 11350 50 500 250 33 60.7 light tan 114 50 50 500 250 34 62.6 light tan 11550 50 500 250 35 64.4 light tan 116 50 50 500 250 10 19.1 very faintgray tint 117 50 50 500 250 15 28.7 light gray 118 50 50 500 250 20 38.2light gray 119 50 50 500 250 25 47.8 very faint tan tint 120 50 50 500250 26 49.7 very faint tan tint 121 50 50 500 250 27 51.6 very faint tantint 122 50 50 500 250 28 53.5 faint tan tint 123 50 50 500 250 29 55.4faint tan tint 124 rt 25 800 800 5 12.7 very faint gray tint 125 rt 25800 800 10 25.3 very faint gray tint 126 rt 25 800 800 15 38.0 lightgray 127 rt 25 800 800 20 50.6 very faint tan tint 128 rt 25 800 800 2563.3 light tan-very faint red tint 129 rt 25 800 800 28 70.8 red-violet130 rt 25 800 800 30 75.9 light violet-red 131 rt 25 800 800 32 81.0violet 132 rt 25 800 800 33 83.5 violet-blue 133 rt 25 800 800 35 88.6light blue-violet 134 rt 25 800 800 5 12.6 very faint gray tint 135 rt25 800 800 10 25.2 very faint gray tint 136 rt 25 800 800 15 37.8 lightgray 137 rt 25 800 800 20 50.4 very faint tan tint 138 rt 25 800 800 2563.0 light tan-very faint red tint 139 rt 25 800 800 26 65.5 faint redtint 140 rt 25 800 800 27 68.0 faint red tint 141 rt 25 800 800 28 70.6light red 142 rt 25 800 800 29 73.1 red-violet 143 rt 25 800 800 30 75.6violet-red 144 rt 25 800 800 5 10.1 very faint gray tint 145 rt 25 800800 10 20.2 very faint gray tint 146 rt 25 800 800 15 30.3 light gray147 rt 25 800 800 20 40.4 light gray 148 rt 25 800 800 23 46.5 lightgray 149 rt 25 800 800 25 50.5 very faint tan tint 150 rt 25 800 800 2652.5 faint tan tint 151 rt 25 800 800 27 54.5 faint tan tint 152 rt 25800 800 28 56.6 light tan 153 rt 25 800 800 29 58.6 light tan 154 rt 25800 800 30 60.6 light tan 155 rt 25 800 800 31 62.6 light tan 156 rt 25800 800 33 66.7 light tan-very faint red tint 157 rt 25 800 800 34 68.7faint red tint 158 rt 25 800 800 35 70.7 faint red tint 159 rt 25 800800 35.5 71.7 light red 160 rt 25 800 800 5 14.7 very faint gray tint161 rt 25 800 800 10 29.3 light gray 162 rt 25 800 800 15 44.0 lightgray 163 rt 25 800 800 18 52.7 faint tan tint 164 rt 25 800 800 20 58.6light tan 165 rt 25 800 800 21 61.5 light tan 166 rt 25 800 800 22 64.5light tan 167 rt 25 800 800 5 14.6 very faint gray tint 168 rt 25 800800 10 29.1 light gray 169 rt 25 800 800 14 40.7 light gray 170 rt 25800 800 15 43.7 very faint tan tint 171 rt 25 800 800 16 46.6 very fainttan tint 172 rt 25 800 800 17 49.5 very faint tan tint 173 rt 25 800 80018 52.4 faint tan tint 174 rt 25 800 800 19 55.3 faint tan tint

[0114] TABLE 2 Reaction SiCl₄ Flow TEOS Flow H₂O Flow Reaction CoatingObserved Example Temperature Rate Rate Rate Time ThicknessRetroreflective No. (° C.) (mL/min) (mL/min) (mL/min) (min) (nm) Color175 50 50 250 500 10 21.1 very faint gray tint 176 50 50 250 500 15 31.7light gray 177 50 50 250 500 20 42.2 light gray 178 50 50 250 500 2552.8 faint tan tint 179 50 50 250 500 30 63.3 light tan-faint red tint180 50 50 250 500 31 65.4 faint red tint 181 50 50 250 500 33 69.6 faintred tint 182 50 50 250 500 34 71.7 light red

[0115] TABLE 3 Coating Time Observed Example No. (min) RetroreflectiveColor 183 20 pale silver/blue 184 40 pale gold/gray 185 60 rust/violet186 67 blue/violet 187 74 blue 188 80 blue-green 189 88 green 190 94yellow-green 191 133  blue-green

[0116] General Procedure for the Preparation of Type II Beads

[0117] Retrochromic beads having microcrystalline regions therein wereprepared by combining in a porcelain milling jar with 1600 g of 1-cmzirconium oxide milling media (obtained under the trade designation “⅜INCH (0.95 CM) RADIUS END ZIRCONIA CYLINDERS,” item no. MEDZOC.37, fromPaul O. Abbe, Inc., Little Falls, N.J.); water; zirconium oxide(obtained under the trade designation “CF-PLUS-HM” from Z-TECH divisionof Carpenter Engineering Products, Bow, N.H.); aluminum oxide (obtainedunder the trade designation “16SG” from ALCOA Industrial Chemicals,Pittsburgh, Pa.); titanium oxide (obtained under the trade designation“KRONOS 1000” from KRONOS, Cranbury, N.J.); wollastonite (obtained underthe trade designation “VANSIL W-30” from R. T. Vanderbilt, Norwalk,Conn.); talc (obtained under the trade designation “SUPRAFINO H” fromLuzenac America, Englewood, Colo.); and either cobalt(II) nitratehexahydrate (obtained under the trade designation “COBALT NITRATECRYSTALS, Lot KMDJ” from Mallinckrodt, Paris, Ky.) andcarboxymethylcellulose (Preparative Examples 1-2, obtained under thetrade designation “CMC 7L2C” from Aqualon Division of HerculesIncorporated, Hopewell, Va.), or iron(III) nitrate nonahydrate(Preparative Examples 3-4, obtained under the trade designation“1110-500” from Fisher, Fairlawn, N.J.). After the respective mixtureswere milled for 3 hours, each was dried to yield a powder cake, whichwas then ground with a mortar and pestle.

[0118] The ground powder was fed into the flame of a hydrogen/oxygentorch obtained from Bethlehem Apparatus Company, Hellertown, Pa., underthe trade designation “BETHLEHEM BENCH BURNER PM2D MODEL B”, hereinafterreferred to as “Bethlehem Burner”. The Bethlehem Burner deliveredhydrogen and oxygen in the inner ring at 8.0 and 3.0 standard liters perminute, respectively, and in the outer ring at 23.0 and 9.8 standardliters per minute, respectively. The melted particles were entrained inthe flame and projected into a water bath where they were rapidly cooled(i.e., quenched). The quenched beads were passed through the flame asecond time and quenched again to improve their optical quality.

[0119] The quenched glass beads were devitrified by placing them inalumina crucibles and subjecting them to heat treatments in a furnace byramping the temperature up from room temperature to the desiredtemperature at a rate of 10° C. per minute, maintaining the desiredtemperature for the desired “hold” time and then allowing the furnace tocool slowly to room temperature. The beads were removed from thecrucibles after they were cooled to room temperature. The weights ofreactants, the hold temperatures and hold times and the ambient-lightand retroreflective colors of the beads of Preparative Examples 1-4 arereported in Table 4 (below). The resultant Type II intrinsicallyretrochromic are useful for preparing articles according to the presentinvention. TABLE 4 Preparative Preparative Preparative PreparativeComponent Example 1 Example 2 Example 3 Example 4 water   160 g   160 g  350 g   350 g ZrO₂ 29.12 g 29.12 g 29.12 g 29.12 g Al₂O₃ 51.31 g 51.31g 51.31 g 51.31 g TiO₂ 62.37 g 62.37 g 62.37 g 62.37 g talc 10.50 g10.50 g 35.71 g 35.71 g wollastonite 48.24 g 48.24 g 24.12 g 24.12 gCo(NO₃)₂.  7.25 g  7.25 g none none 6H₂O Fe(NO₃)₃. none none  5.06 g 5.06 g 9H₂O carboxymethyl-  3.0 g  3.0 g none none cellulose hold 940°C. 985° C. 975° C. 1000° C. temperature hold time 30 min 30 min 30 min60 min color in gray-blue gray-blue off-white off-white ambient lightretroreflective silver-blue brown yellow orange- color brown

EXAMPLE 192

[0120] A kit of intrinsically retrochromic beads was prepared asfollows. Intrinsically retrochromic beads (approximately 1 g each),prepared according to Examples 37-49 were dispensed separately intothirteen 2 mL (0.5 dram) glass vials with caps (available under thetrade designation “CLASS A CLEAR GLASS THREADED VIALS WITH RUBBER-LINEDCLOSURES” from Fisher Scientific, Pittsburgh, Pa.). The vials wereplaced into slots of a cardboard container (available under the tradedesignation “WHEATON LAB FILE STORAGE SYSTEM” from Fisher Scientific).

EXAMPLE 193

[0121] A kit of intrinsically retrochromic beads was prepared asfollows. Intrinsically retrochromic beads (approximately 1 g each),prepared according to Examples 37-49 were dispensed separately intothirteen 2 mL (0.5 dram) glass vials with caps (obtained from FisherScientific, Pittsburgh, Pa., under the trade designation “CLASS A CLEARGLASS THREADED VIALS WITH RUBBER-LINED CLOSURES”). The vials wereaffixed to cardboard (commercially available under the trade designation“REGULAR MAT BOARD CR-SRM 989 RAVEN BLACK”, from Crescent Cardboard Co.LLC, Wheeling, Ill.) using transparent tape (obtained from the 3MCompany under the trade designation name “SCOTCH BRAND TRANSPARENT TAPE600”).

[0122] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and spirit of this invention, and it should be understood thatthis invention is not to be unduly limited to the illustrativeembodiments set forth herein.

What is claimed is:
 1. A method of making intrinsically retrochromicbeads comprising: providing a fluidized bed of beads; depositing aconcentric optical interference layer onto the respective surface ofeach of a plurality of beads to form a plurality of coated beads; andremoving a first portion of coated beads from the fluidized bed, whereinthe concentric optical interference layers comprising the first portionhave a first average thickness, and wherein the first portion comprisesintrinsically retrochromic beads.
 2. The method of claim 1, wherein atleast one of the coated beads has a first refractive index, wherein theconcentric optical interference layer of said at least one bead has asecond refractive index, and wherein the second refractive index isgreater than the first refractive index.
 3. The method of claim 1,wherein at least one of the coated beads has a first refractive index,wherein the concentric optical interference layer of said at least onebead has a second refractive index, and wherein the second refractiveindex is less than the first refractive index.
 4. The method of claim 1,further comprising the step of monitoring the deposition of theconcentric optical interference layer by viewing the coated beads inretroreflective mode.
 5. The method of claim 1, wherein the concentricoptical interference layer comprises a metal oxide.
 6. The method ofclaim 5, wherein the metal oxide comprises at least one of silica,titania, or alumina.
 7. The method of claim 1, wherein depositingcomprises chemical vapor deposition.
 8. The method of claim 7, whereindepositing further comprises hydrolysis.
 9. The method of claim 1,wherein the beads comprise a colorant.
 10. The method of claim 1,further comprising removing a second portion of coated beads from thefluidized bed, wherein the concentric optical interference layerscomprising the second portion have a second average thickness, andwherein the second portion comprises intrinsically retrochromic beads,wherein the second average thickness is greater than the first averagethickness.
 11. The method of claim 10, further comprising the step ofmonitoring the deposition of the concentric optical interference layerby viewing the coated beads in retroreflective mode.
 12. The method ofclaim 10, wherein the concentric optical interference layer comprises ametal oxide.
 13. The method of claim 12, wherein the metal oxidecomprises at least one of silica, titania, or alumina.
 14. The method ofclaim 10, wherein depositing comprises chemical vapor deposition. 15.The method of claim 10, wherein depositing further comprises hydrolysis.16. The method of claim 10, wherein the first and second portions havedifferent retroreflective colors.
 17. The method of claim 10, whereinthe beads comprise a colorant.
 18. The method of claim 10, furthercomprising removing a third portion of coated beads from the fluidizedbed, wherein the concentric optical interference layers comprising thethird portion have a third average thickness, and wherein the thirdportion comprises intrinsically retrochromic beads, wherein the thirdaverage thickness is greater than the second average thickness.
 19. Themethod of claim 18, further comprising the step of monitoring thedeposition of the concentric optical interference layer by viewing thecoated beads in retroreflective mode.
 20. The method of claim 18,wherein the first, second, and third portions have differentretroreflective colors.
 21. The method of claim 20, wherein thedifferent retroreflective colors comprise a retroreflective colorpalette.
 22. A kit comprising a first plurality of intrinsicallyretrochromic beads having a first average retroreflective color, asecond plurality of intrinsically retrochromic beads having a secondaverage retroreflective color, and a third plurality of intrinsicallyretrochromic beads having a third average retroreflective color, whereinsaid retroreflective colors are distinct and comprise a retroreflectivecolor palette.
 23. The kit of claim 22, wherein the first, second, andthird pluralities of intrinsically retrochromic beads are separatelyconfined within at least one reclosable container.
 24. The kit of claim23, wherein the pluralities of intrinsically retrochromic beads areconfined within respective reclosable containers.
 25. The kit of claim24, wherein the holder is adapted to receive said reclosable containers.26. The kit of claim 22, wherein the retroreflective color palettecomprises at least three of violet, blue, orange, yellow, red, or greenretroreflective colors, if viewed using white light illumination. 27.The kit of claim 22, wherein the retroreflective color palette comprisesyellow, magenta, and cyan retroreflective colors, if viewed using whitelight illumination.
 28. The kit of claim 22, wherein said pluralitieshave substantially the same average color if viewed innonretroreflective mode.
 29. The kit of claim 22, wherein at least oneof the intrinsically retrochromic beads comprises a core and aconcentric optical interference layer.
 30. The kit of claim 29, whereinthe index of refraction of the concentric optical interference layer isless than the index of refraction of the core.
 31. The kit of claim 29,wherein the index of refraction of the concentric optical interferencelayer is greater than the index of refraction of the core.
 32. The kitof claim 29, wherein at least one of the intrinsically retrochromicbeads has microcrystalline regions.
 33. The kit of claim 29, furthercomprising a fourth plurality of retroreflective beads wherein saidfourth plurality is distinct from said first, second, and thirdpluralities and wherein said fourth plurality is not intrinsicallyretrochromic.
 34. The kit of claim 29, wherein the retroreflective colorpalette comprises at least three of violet, blue, orange, yellow, red,or green retroreflective colors if viewed using white lightillumination.
 35. The kit of claim 29, wherein the palette comprisesyellow, magenta, and cyan retroreflective colors, if viewed using whitelight illumination.
 36. The kit of claim 29, wherein said pluralitieshave substantially the same average color if viewed innonretroreflective mode.
 37. A method of preparing a plurality ofretroreflective beads having a custom retroreflective color comprising:providing a first plurality of intrinsically retrochromic beads having afirst average retroreflective color; providing a second plurality ofintrinsically retrochromic beads having a second average retroreflectivecolor; and combining, with mixing, said first and second pluralities ofretrochromic beads.
 38. The method of claim 37, further comprising:providing a third plurality of intrinsically retrochromic beads having athird retroreflective color; and combining, with mixing, the thirdplurality of intrinsically retrochromic beads with said first and secondpluralities of retrochromic beads.